For recognizing a fingerprint of a person's finger by an optical fingerprint imaging system, the finger is placed on a prism surface whereby the incident light will be absorbed by the ridge of the finger on the finger receiving surface, while the incident light will be internally reflected from the finger receiving surface underneath a valley of the finger, thereby forming fingerprint images having dark areas of the reflection image corresponding to the ridges and having the light areas corresponding to the valleys of the fingerprint.
However, a dry finger has a poor cohesion to the finger receiving surface of the fingerprint recognizing system, the incident light from an illuminator may not be completely absorbed by the ridges of the dry finger and will be partially internally reflected from the finger receiving surface, thereby causing a vague image of the fingerprint and deteriorating the fingerprint identification.
A conventional method to overcome the above-mentioned defect has been tried by moistening the finger with water or alcohol in order to have a closer contact of the finger on the prism surface for making a clear fingerprint image.
Still, the moistening with water or alcohol on the finger has the following drawbacks:
1. The water or alcohol has a good fluidity and is easily volatile to make the contacting areas unclear between the finger and the prism surface. For instance, the water or alcohol may flow from a ridge to a valley to influence the total internal reflection at the valley when touching the finger on the prism surface, thereby deteriorating the image quality of the fingerprint recognition. PA1 2. Even though the water may be soaked in a cotton pad, the water quantity as moistened on the finger from the pad can not be well controlled. Too much water may still contaminate the prism surface and may influence the imaging precision during fingerprint recognition. PA1 3. The wet finger when moistened with water or alcohol may cause dislike feeling for the person to be detected. Meanwhile, the water or alcohol pad should be always maintained or replaced with fresh one for hygienic purpose, causing inconvenience for the maintenance of the fingerprint imaging device.
The present inventor has found the drawbacks of the conventional fingerprint recognition systems and invented the present finger-moistening means for fingerprint recognizing apparatus.